#composites, #sustainability, #fibers, #resins, #bio-based, #natural_fiber, #plant-based, #cellulose, #lignin, #bio_based_epoxy
I want to go back to talking a bit about lignin this week. It seems the more I dig into this stuff the more interesting it gets. Last week I touched on both cellulose and lignin because they are the most abundant organic polymers on the planet – because they are in all plants. It is actually the breakdown of cellulose and lignin deep in the earth’s crust that gives us petroleum – the stuff we are using as fuel today. And there is as much as 70 million tons of this stuff that is available for use today in forest and agricultural waste. Certainly, we ought to be able to do something with this stuff – it is cheap, abundant, available, and doesn’t come out of an oil well.
But first, I need to talk a little about the chemistry at work here. The pic that you see above is the chemical structure of the three what are called “monolignols”. All three of these organic compounds are phenolics because they are based on phenol – the carbon atom hexagon with an O-H group attached to it. This is the basic structure of the starting point for nearly all composite resins and most carbon fibers. I’ve written quite a bit about the structure of the benzene ring and how it provides stiffness and strength to both carbon fibers and the composite resins that stick the fibers together to make a composite.
These Lignols do the same thing for plants. Plants use them to stick the cellulose fibers together to provide rigidity and strength to the plant. And these three monolignols have very different properties and are used by the plant world in different ways. The one on the left – coniferyl alcohol – is mostly used by softwoods like pine, fir, redwood, spruce, etc. This form of lignin has a bit more flexibility and is not as hard as is the sinapyl alcohol which is the predominant lignol that hardwoods like maple and oak use.
So, what is the possibility that we can use this waste material from the paper processing industry as a feedstock for composites? Actually, the potential is quite large. Researchers in Europe, China, Brazil, and the US have been working on this for a few years now, and have developed a fairly broad range of plastics and polymers that can be spun into fibers. The pic below shows what the eventual plan (dream?) is for lignin based carbon fiber.
However, no lignin based carbon fibers have made it into high volume production. This is because just using what is called “kraft lignin” – the typical byproduct of the paper making industry where the lignin is sulfated to make it dissolve in water – makes a carbon fiber that is more brittle and not as strong or stiff as PAN-based carbon fibers.
Since the initiation of lignin-based carbon fiber research, different researchers have added thermoplastics like polypropylene, PET, and even poly(ethylene oxide) or PEO. And, to enable a completely sustainable carbon fiber, a plant-based poly(lactic acid) or PLA was mixed with lignin as an attempt to make a carbon fiber precursor. Each of these produced fibers, but they all had inferior properties to PAN based carbon fiber.
Fortunately, that research has continued, and there are some promising developments on the horizon. A group of researchers in China have developed a corn-derived lignin bio oil that they have polymerized with acrylonitrile (the stuff that is used to make carbon fibers) and have spun this into fibers using an electrospinning process. This process is common in the composites industry, and apparently this combination makes for a fairly high quality fiber. Still not quite the same as the petroleum-based PAN carbon fiber that is the standard of the industry today. So, lignin-based high performance carbon fiber is likely on the fairly short term horizon. This is very good news for the composites industry because the resulting fiber will not only be entirely plant-based, it will also be significantly less expensive than petroleum-based PAN fiber, and it will not be subject to the price and supply swings that are the norm in the oil patch.
There are more developments ongoing as well to produce a high quality carbon fiber from lignin that will bear fruit in the relatively short term – at least as these things go which means 5 to 10 years from now. But, what about resins from lignin. I mentioned briefly above that researchers in China were using a lignin based bio-oil in their carbon fiber developments.
How about using lignin as a feedstock for an epoxy resin? It turns out that this is in the works. Several researchers have been working on this for a while now, and there are several promising developments. There have been several researchers working on this problem for some time. Since lignin is a phenolic compound, using one or the other lignols (like those shown in the lead pic in this post) to make an epoxy is fairly straightforward chemistry.
This is a figure from a review paper about this process, and it shows some of the chemistry that has been used to turn lignin into an epoxy resin. Note the two toward the bottom here – these are lignin molecules polymerized with our old friend Bisphenol-A. The top one of course is an epoxy made using just the lignin phenolic (the red triangle is the epoxide group – just FYI). But, the bottom line here is that since we can synthesize Bisphenol-A from many different plant sources – including lignin albeit with quite a bit more processing than with other plant oils – fully plant-based high performance epoxies are on the horizon as well. And using lignin and other plant oils instead of petroleum also means that these resins are not only sustainable and potentially can be made bio-degradable, they will also be less expensive than petroleum-based resins.
This is very good news, and demonstrates why I am so excited to see some of the upcoming developments for what has to date been considered waste from both the forest products and agricultural industries. The second most abundant bio-polymer available naturally – lignin – has a very bright future in the composites industry and is one of the keys to unlocking the composites sustainability puzzle.
That’s about enough for this week. I am going to be reminding everyone right here each week that I will be presenting one more paper toward the end of this year. And again, this one is about sustainability of composites focusing on carbon fiber. And as most of you that have been reading my posts, this is a subject that you know is a passion of mine. This one will be at the Carbon Fiber Conference in Salt Lake City being put on by Composites World (https://www.carbonfiberevent.com/). In that presentation I will focus on current work in sustainability of carbon fiber in particular. I’m of course going to talk about new fibers and fiber precursors made from plants, so again, focusing on closing the circle.
And, finally, for those of you that have not heard, my book has been published and has been for sale since August of this year. The best place to get one is to go to my website and buy one. I will send you a signed copy for the same price you would get charged on Amazon, except that I charge $8 shipping. Anyway, here’s the link to get your signed copy: https://www.nedpatton.com/product-page/the-string-and-glue-of-our-world-signed-copy. And as usual, here’s a picture of the book, for those of you just tuning in.
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